Hydrogen volcanoes may increase habitability of exoplanets

Artist's concept of a possible surface of planet TRAPPIST-1f(Credit: NASA/Caltech)

Does TRAPPIST-1 have three potentially habitable planets or four? New research by scientists at Cornell University led by Ramses Ramirez, indicates that number four is a possibility if there are hydrogen volcanoes on it. According to the astronomers, worlds where hydrogen spews out from volcanic vents could enjoy a greenhouse effect that would warm their atmosphere enough to sustain life.

When the discovery of seven Earthlike planets with three occupying the habitable zone of the Jupiter-sized red dwarf star TRAPPIST-1 was announced last week, it caused a minor sensation in the scientific world. The habitable, or Goldilocks, zone is that band of orbits where it is not too hot and not too cold, but just right for a planet to potentially have liquid water on its surface.

In our solar system, the habitable zone extends from about the orbit of Venus to slightly beyond that of Mars, so three planets in the system could potentially support life and one definitely does. While exoplanets orbiting other stars have been found in their local habitable zone, finding seven Earth-like planets revolving around a small, cold dwarf star with three in the right place is the astronomical equivalent of drawing to an inside straight.

Not surprisingly, this makes scientists think that there may be more habitable planets than previously believed and the new Cornell study indicates that the habitable zone of TRAPPIST-1 and other stars may extend farther out than previously thought.

One thing that keeps the Earth at a habitable temperature is the greenhouse effect of water vapor, carbon dioxide, and other gases in the atmosphere. Remove their effect and the surface temperatures would be more like Mars. According to Cornell, if an otherwise icy exoplanet had an atmosphere of hydrogen, carbon dioxide, and water vapor, it, too, would enjoy a greenhouse effect, which could warm it enough for liquid water to exist and effectively extend the habitable zone by 30 to 60 percent. In addition, the planet would contain biosignatures, such as methane or ozone, in its atmosphere that our next generation of space telescopes could detect.

The only snag is that hydrogen is a gas that does not like to stick around. In our solar system, only gas giants like Jupiter have enough gravity to keep most of their hydrogen in the atmosphere. On small rocky worlds like Earth, any hydrogen simply floats up and off into space. Cornell's research indicates that if an Earth-like planet had volcanoes that belched out hydrogen at a steady rate, it could maintain a high enough level of the gas in the atmosphere to sustain a greenhouse effect.

If such a planet existed in our solar system, it could orbit as far as 2.4 times the distance from the Sun as the Earth, which would place it roughly in the asteroid belt between Mars and Jupiter. For TRAPPIST-1, if the outermost seventh planet has hydrogen volcanoes, it means that the system has four potentially habitable worlds instead of three.

"Finding multiple planets in the habitable zone of their host star is a great discovery because it means that there can be even more potentially habitable planets per star than we thought," says Lisa Kaltenegger, Cornell professor of astronomy and director of the Carl Sagan Institute. "Finding more rocky planets in the habitable zone – per star – increases our odds of finding life."